Rectangular Concrete Tank: Stability Forces

[MrEngineerUS]

I am reviewing the sliding stability of a 16'x14'x17'-deep concrete tank. Essentially, it will need to behave as a retaining wall during a possible future installation of an adjacent tank. There will be full height soil on one side and full excavation on the opposite side of the tank.

When I check the sliding stability for this case I get a S.F. = 0.74. Increasing the slab doesn't add enough weight to increase the S.F. by much.

I am hoping to use the friction between the soil and the SIDEWALLS to help pump-up my S.F. > 1.5. However, I can't find any information concerning how to adequately calculate this resistance.

My guess would be to use the Friction Angle for Dissimilar Materials from AASHTO (typically used in calculating the vertical component of Coulomb lateral earth pressures) to get a corresponding friction coefficient for the walls. Then, multiply the linearly varying lateral soil pressure normal to the sidewalls of the tank to get a linearly varying friction resistance for the orthogonal direction.

Thoughts? Thanks!

 

[JedClampett]

I'd be careful about any tricky methods of increasing the F.S. If you can't increase the mass or use the soil on the heel, try a key and use passive pressure.

 

[JStephen]

Could you maybe extend one wall several feet past the corners on each end?

 

[TehMightyEngineer]

Since this is a known temporary condition and isn't likely to exist for very long can you justify taking out some conservative numbers in your lateral soil load calculations? As for friction on the sidewalls I'd only use it if you had no other way but I think you have other options worth exploring first.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[graybeach]

I don't think it is a good idea because the soil on the sides could be excavated at some future date.

 

[MrEngineerUS]

I have considered adding a key. Extending the walls to engage more soil is an interesting idea and worth exploring as well.

It does seem like this type of shear resistance should be allowed to be used though. When I picture the system in my head that really is a lot of friction on the orthogonal walls that is just being neglected (i.e. treated like frictionless surfaces when they are anything but). It would probably be prudent to only consider half of the wall lengths as friction resistance to account for possible soil disruption near the excavation side of the tank if I were to go this route, however.

Thanks for the insights!

 

[BUGGAR]

Any chance you could just fill it with water and pump it out later?

 

[TehMightyEngineer]

Oh good idea BUGGAR; if water doesn't work you could pile in a ton of sandbags inside or similar.

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[PEinc]

See duplicate posting in the Earth Retention Engineering Forum.

http://www.PeirceEngineering.com

 

[BAretired]

To be specific, it is thread255-393897.
Duplicate threads are sometimes frowned upon on Eng-Tips.

BA

 

[MrEngineerUS]

Sorry about the duplicate thread here; I'll keep that in mind in the future for sure! Thank you for all the guidance and I unless I can find some existing rational for considering side wall friction I will likely either recommend the existing structure be braced during future installation or (if the client really, really wants me to) design it to be stable without the side wall friction (shear key and extended toe, most likely).